Septic tanks operate by allowing sewage to be digested in the tank for an appropriate residence time before fluid which has been subject to digestion in the tank overflows from the tank into a distribution system in surrounding soil. After a period of time, there is an inevitable build-up of sludge at the bottom of the septic tank. As the level of sludge in the septic tank rises, the residence time of sewerage entering the tank for digestion begins to decrease. When the sludge level becomes high, the residence time of sewerage may be so short that there is insufficient time for adequate digestion. As a result, inadequately digested sewerage from the overflow outlet of the tank which may even contain suspended matter is fed to the surrounding liquid distribution system. The dirty effluent can cause the distribution lines to clog with the result that the surrounding area may become contaminated with dangerous biological material. This biological material can even seep into nearby creeks and rivers making them unsafe for recreational or drinking purposes.
Scum, a lightweight layer typically including a substantial oil component may also build up as a surface layer with time. Excess scum may also hinder digestion and lead to premature overflow of inadequately digested material.
In order to reduce the likelihood of contamination of surrounding areas and water courses by septic tanks, there is a mandatory legal requirement in the State of Victoria, Australia that septic tanks be emptied at least once every three years to remove sludge and/or scum. However, in practice despite this mandatory legal requirement, most septic tanks are not emptied for far longer periods.
In most instances, sludge or scum build-up in a typical septic tank will not be unacceptable in the relatively short time span of three years. However, because of the variable nature of usage of different septic tanks and the fact that there is no adequate monitoring system, the relatively short mandatory three year time span for emptying septic tanks is necessary to cover the unusual situation where some septic tanks require emptying within such a short period. The risks of not following such a regime, such as threats to health and the expense of redoing the distribution lines when they are clogged, are otherwise too great.
Thus there is a need for a method of monitoring when a septic tank needs to be inspected for excess scum or sludge so as to avoid the need for mandatory emptying every three years or such other time as is set by legislation.